Despite new technologies that significantly aid in reducing exhaust emissions from internal combustion diesel engines, NOx and particulate matter (PM) emissions are still a subject of environmental concern. The term “NOx” includes the mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). NOx is a particular concern for diesel engines because of their lean burn operation, but some gasoline engines include lean burn operation as well.
Emission standards for vehicle engines are becoming increasingly stringent, and it is difficult to meet governmental emissions regulations by merely relying only on improvements to the engine itself. Thus, today's engines continue to require some sort of exhaust gas aftertreatment system.
For diesel and other lean burn engines, various exhaust aftertreatment devices have played an essential role in engine emission technologies. Diesel particulate filters (DPF's) are used for particulate matter (PM) control. Selective catalytic reduction (SCR) devices, lean NOx traps and NOx adsorbers are used for NOx control.